US Ambassador, Chris Stevens killed in Libya September 12, 2012. Photo: cnn.com

An Arabic speaker who loved Libya and understood it deeply, Stevens died along with three other Americans when an angry mob stormed the U.S. Consulate in Benghazi. He was the first U.S. ambassador to be killed in the line of duty since 1979.

The consulate was one of several American diplomatic missions in the Middle East to face protests Tuesday after the release online of a film mocking Islam and depicting the Muslim prophet Mohammed as a child molester, womanizer and ruthless killer.

But U.S. sources told CNN on Wednesday that the Benghazi attack was planned in advance, and the attackers used the protest outside the consulate as a diversion. The sources could not say whether the attackers instigated the protest or merely took advantage of it, and they say they don't believe Stevens was specifically targeted.

The consulate was one of several American diplomatic missions in the Middle East to face protests Tuesday after the release online of a film mocking Islam and depicting the Muslim prophet Mohammed as a child molester, womanizer and ruthless killer.

But U.S. sources told CNN on Wednesday that the Benghazi attack was planned in advance, and the attackers used the protest outside the consulate as a diversion. The sources could not say whether the attackers instigated the protest or merely took advantage of it, and they say they don't believe Stevens was specifically targeted.

A senior U.S. official familiar with the details of the attack said a grenade set the building ablaze, leaving the Americans facing both a fire inside and attackers outside. Stevens and the others who died were separated from the rest of the staff while trying to escape to the roof of the building and succumbed to smoke inhalation, the senior official said.

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The official said there were several "valiant but unsuccessful" attempts to get back into the building and rescue them.

Meanwhile, a London think tank with strong ties to Libya speculated Wednesday that Stevens was actually the victim of a targeted al Qaeda revenge attack. The assault "came to avenge the death of Abu Yaya al-Libi, al Qaeda's second in command killed a few months ago," the think tank Quilliam said Wednesday.

It was "the work of roughly 20 militants, prepared for a military assault," the think tank said, noting that rocket-propelled grenade launchers do not normally appear at peaceful protests, and that there were no other protests against the film elsewhere in Libya.

The planned attack came in two waves, one which prompted U.S. officials to leave the consulate for a secure location. The second wave was directed at the place of retreat, Quilliam said, citing unnamed sources on the ground in Benghazi and abroad.

In Washington, President Barack Obama said the violence was completely unwarranted and vowed that "justice will be done."

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"Since our founding, the United States has been a nation that respects all faiths. We reject all efforts to denigrate the religious beliefs of others," he said. "But there is absolutely no justification for this type of senseless violence -- none."

Secretary of State Hillary Clinton dispatched Stevens to Benghazi to be the American link to rebel forces battling to overthrow Gadhafi in 2011.

"He arrived on a cargo ship in the port of Benghazi and began building our relationships with Libya's revolutionaries," Clinton said Wednesday. "He risked his life to stop a tyrant, then gave his life trying to build a better Libya."

She identified a second victim as Sean Smith, a Foreign Service information management officer who was a 10-year veteran of the State Department, a husband and a father of two.

The two other victims have not been named. Clinton said the government was working to contact their families.

"The mission that drew Chris and Sean and their colleagues to Libya is both noble and necessary, and we and the people of Libya honor their memory by carrying it forward. This is not easy," Clinton said. But she added, "We must be clear-eyed even in our grief."

"This was an attack by a small and savage group, not the people or government of Libya. Everywhere Chris and his team went in Libya, in a country scarred by war and tyranny, they were hailed as friends and partners. And when the attack came yesterday, Libyans stood and fought to defend our post. Some were wounded. Libyans carried Chris's body to the hospital, and they helped rescue and lead other Americans to safety."

Libyan Prime Minister Abdurrahim el-Keib apologized "to the American people and the government, and also to the rest of the world" for the "cowardly, criminal act."

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About 50 U.S. Marines from a rapid reaction force headed to Libya after the attack, U.S. officials said Wednesday.

Libyan Deputy Interior Minister Wanis al-Sharif blamed the violence on people loyal to deposed dictator Gadhafi.

Consulate security staff opened fire after they heard gunfire outside the mission, al-Sharif said.

"This led to more anger, and this is when the consulate was stormed," he said. "Criminals managed to get in and they burned and ransacked the consulate."

The consulate was badly damaged and was being looted Wednesday, said a contractor working at the mission, who asked not to be named for security reasons. He said he saw the bodies of all four Americans on the street Wednesday morning.

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Libyan Deputy Prime Minister Mustafa Abushagur said Stevens was "a friend of Libya, and we are shocked at the attacks on the U.S. consulate in Benghazi."

"I condemn these barbaric acts in the strongest possible terms. This is an attack on America, Libya and free people everywhere," he said on Twitter.

The contractor in Benghazi said he could hear rocket-propelled grenade attacks Tuesday night.

Libyans were also killed, the contractor said, saying the victims were shot on the spot.

The bodies of the four Americans were at Benghazi airport, the contractor said, citing the Libyan minister of foreign affairs and a top immigration official in Benghazi.

Friends say Stevens loved Libya and had a deep affinity for the Libyan people, enjoying heading out into the field and getting to know people.

A speaker of Arabic and French, he was among the first U.S. diplomats sent to Libya in 2007 when the United States resumed ties with the Gadhafi regime.

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The last time an American ambassador was murdered was in 1979, when the envoy to Afghanistan, Adolph Dubs, was kidnapped and killed during an attempt to rescue him, according to State Department records.

Stevens is the sixth U.S. ambassador to die by violence in the line of service. Two others have been killed in plane crashes.

The United States was taking added security measures to protect its citizens worldwide after Tuesday's attacks.